1. Field of the Invention:
The present invention generally relates to a standard illuminant apparatus and, more particularly, to a standard illuminant apparatus capable of providing standard optical property of LED for measurement.
2. Description of the Prior Art:
Since the light emitting diode (LED) is advantageous in long lifetime, low power consumption, shock resistance, mass production, light weight, compactness and fast response, it has been widely used in general lighting, backlight modules for liquid-crystal displays (LCD's) and automotive illumination. In general, quality parameters for the LED include luminous intensity, luminous flux, and chromaticity. However, the time-dependent optical property of LED and the special light-emitting distribution lead to poor precision in measurement. The optical property of LED depends strongly on the temperature. For example, heat is generated in LED chip as light is emitted from the LED. Such heat causes the luminous intensity to change. As a result, the luminous intensity of light from the LED measured in a period of time becomes unstable and unreliable. Moreover, the result is different even if the same LED is measured several days later. In other words, the reproducibility is poor.
Consequently, parameters of the luminous intensity are questionable especially when the same LED is measured by different testers. Generally, an LED with better quality is used as a standard for calibration. However, no matter how the LED is manufactured, it exhibits poor reliability and reproducibility.
Moreover, a conventional temperature-controlled LED is also used as standard light source. By detecting the temperature around the LED, a current is provided to the LED so as to improve the reliability of the LED. However, the temperature-controlled LED still suffers from poor reproducibility.
Furthermore, the LED can be implemented in many ways, for example, a surface-mounted LED light shape control module, a lamp-type LED light shape control module, an array-type LED light shape control module or an side-emitting LED light shape control module. The manufacturing cost of the standard illuminant apparatus is increased if the aforesaid LED's are modulated by a complicated temperature-controlled apparatus.